To cope with a request of high-speed responsiveness.

When a damping time constant D is changed into a state of D=0, a sampling period Ts is automatically changed from 100 ms to 50 ms, and a sampling time ts is automatically changed from 60 ms to 30 ms. Hereby, when the damping time constant D is changed to the state of D=0, a processing unit samples an electrical signal corresponding to a pressure difference of fluid detected by a differential pressure sensor in each sampling period Ts=50 ms during a sampling time ts=30 ms, and performs a variety of operation processing such as a linearization operation or a square root operation to the sampled electrical signal, to thereby find a measurement value P of the differential pressure. In the operation processing, the processing unit 3 does not perform a damping operation.